Convection is the transfer of thermal energy through the movement of fluids (gases or liquids), either naturally due to density differences or forced by mechanical means. Convection is the primary heat transfer mechanism in HVAC systems, responsible for delivering conditioned air and transferring heat between air and coil surfaces.
Natural and Forced Convection
Natural convection occurs without fans, as warmer air rises and cooler air sinks, creating circulation patterns. Forced convection uses fans and blowers to move air across heat exchangers, increasing heat transfer rates by 5 to 15 times compared to natural convection. Heat transfer rate is expressed as Q = h × A × ΔT, where h is the convection coefficient (typically 2-20 BTU/hr·ft²·°F for natural convection and 10-100 for forced convection).
HVAC Applications
Furnace heat exchangers, air conditioner coils, and fan coil units all rely on forced convection to transfer thermal energy to conditioned air. Duct design ensures proper air velocity (typically 700-900 CFM in residential systems) to maximize convection efficiency without excessive noise or ductwork pressure drop.